What size grounding conductor for 5000amp Feeder?

[mbrooke]

I came across two separate feeder schedules, and to be honest I am having a brain freeze on which EGC is correct for 5000amps 2-350 or 2 4/0.

 

[jumper]

See T250.122 and the article section (F)

 

[infinity]

I came across two separate feeder schedules, and to be honest I am having a brain freeze on which EGC is correct for 5000amps 2-350 or 2 4/0.







View attachment 20842 View attachment 20843

Neither. Look at Jumper's references and see if you can find where it says you can parallel two smaller EGC's to make a larger one.

 

[mbrooke]

See T250.122 and the article section (F)

Thanks.

(F) Conductors in Parallel. Where conductors are installed
in parallel in multiple raceways or cables as permitted
in 310.10(H), the equipment grounding conductors, where
used, shall be installed in parallel in each raceway or cable.
Where conductors are installed in parallel in the same raceway,
cable, or cable tray as permitted in 310.10(H), a single
equipment grounding conductor shall be permitted. Equipment
grounding conductors installed in cable tray shall meet
the minimum requirements of 392.10(B)(1)(c).
Each equipment grounding conductor shall be sized in
compliance with 250.122.

So I would need 12 conductors provided the circular mil area equals 700kcmil... but not smaller than 1/0 when all is said and done... correct in thinking like this? The way its worded it almost sounds like I need 700 for each conduit.

 

[infinity]

Thanks.

So I would need 12 conductors provided the circular mil area equals 700kcmil... but not smaller than 1/0 when all is said and done... correct in thinking like this? The way its worded it almost sounds like I need 700 for each conduit.

No, each parallel raceway requires a full size EGC from T250.122. You cannot parallel smaller condcutors to make a larger one.

 

[mbrooke]

No, each parallel raceway requires a full size EGC from T250.122. You cannot parallel smaller condcutors to make a larger one.

Ok.

What is the theory behind that though? Just curious.

 

[infinity]

Ok.

What is the theory behind that though? Just curious.

I'm not sure but seems to me like a waste of materials and money especially when you're using a metallic raceway that qualifies as an EGC.

 

[mbrooke]

I'm not sure but seems to me like a waste of materials and money especially when you're using a metallic raceway that qualifies as an EGC.

This has to be the daftest rule in the NEC Can I at least bet on 250.122 A :angel:

(A) General. Copper, aluminum, or copper-clad aluminum
equipment grounding conductors of the wire type shall not
be smaller than shown in Table 250.122, but in no case
shall they be required to be larger than the circuit conductors
supplying the equipment. Where a cable tray, a raceway,
or a cable armor or sheath is used as the equipment
grounding conductor, as provided in 250.118 and
250.134(A), it shall comply with 250.4(A)(5) or (B)(4).

 

[infinity]

This has to be the daftest rule in the NEC Can I at least bet on 250.122 A :angel:

With 12 sets of 600 kcmil the EGC in each raceway would need to be 700 kcmil or larger.

 

[kwired]

This has to be the daftest rule in the NEC Can I at least bet on 250.122 A :angel:

(A) General. Copper, aluminum, or copper-clad aluminum
equipment grounding conductors of the wire type shall not
be smaller than shown in Table 250.122, but in no case
shall they be required to be larger than the circuit conductors
supplying the equipment. Where a cable tray, a raceway,
or a cable armor or sheath is used as the equipment
grounding conductor, as provided in 250.118 and
250.134(A), it shall comply with 250.4(A)(5) or (B)(4).

I would think the logic is that if you have a ground fault in one raceway the EGC must be able to carry enough current to reliably cause the 5000 amp OCPD to operate, and that would be sized per 250.122. Also sort of makes sense, though I don't know that all will interpret this way, that an individual EGC wouldn't need to be any larger than the largest ungrounded conductor in the same raceway/cable - it is not going to take more current away from the fault location then the ungrounded conductor can deliver.

(A) General. Copper, aluminum, or copper-clad aluminum
equipment grounding conductors of the wire type shall not
be smaller than shown in Table 250.122, but in no case
shall they be required to be larger than the circuit conductors
supplying the equipment. Where a cable tray, a raceway,
or a cable armor or sheath is used as the equipment
grounding conductor, as provided in 250.118 and
250.134(A), it shall comply with 250.4(A)(5) or (B)(4).

 

[mbrooke]

With 12 sets of 600 kcmil the EGC in each raceway would need to be 700 kcmil or larger.

Bummer.

What about the neutral conductors? What if its a service and there is only a grounded conductor?

 

[mbrooke]

I would think the logic is that if you have a ground fault in one raceway the EGC must be able to carry enough current to reliably cause the 5000 amp OCPD to operate, and that would be sized per 250.122. Also sort of makes sense, though I don't know that all will interpret this way, that an individual EGC wouldn't need to be any larger than the largest ungrounded conductor in the same raceway/cable - it is not going to take more current away from the fault location then the ungrounded conductor can deliver.

(A) General. Copper, aluminum, or copper-clad aluminum
equipment grounding conductors of the wire type shall not
be smaller than shown in Table 250.122, but in no case
shall they be required to be larger than the circuit conductors
supplying the equipment. Where a cable tray, a raceway,
or a cable armor or sheath is used as the equipment
grounding conductor, as provided in 250.118 and
250.134(A), it shall comply with 250.4(A)(5) or (B)(4).

I would agree, but what about your ungrounded conductors? Its possible to have them not be protected only by being smaller than table 250.122. I think if that was the case, the code would forbid paralleling all conductors if they were smaller than 250.122. Not saying you are wrong- its possible that they would rather risk burning up the conductors then having a life frame when comparing evils.


BTW, I believe in the Mandela effect and alternate realities now Maybe slipped into another dimension when walking at night- and I am being searched for in the other by loved ones. Either that all that funky stuff I did years back is catching up to me. I remember parallel EGCs being sized smaller than table 250.122 provided the total count hit the cross sectional area of 250.122.

 

[infinity]

I would think the logic is that if you have a ground fault in one raceway the EGC must be able to carry enough current to reliably cause the 5000 amp OCPD to operate, and that would be sized per 250.122. Also sort of makes sense, though I don't know that all will interpret this way, that an individual EGC wouldn't need to be any larger than the largest ungrounded conductor in the same raceway/cable - it is not going to take more current away from the fault location then the ungrounded conductor can deliver.

(A) General. Copper, aluminum, or copper-clad aluminum
equipment grounding conductors of the wire type shall not
be smaller than shown in Table 250.122, but in no case
shall they be required to be larger than the circuit conductors
supplying the equipment. Where a cable tray, a raceway,
or a cable armor or sheath is used as the equipment
grounding conductor, as provided in 250.118 and
250.134(A), it shall comply with 250.4(A)(5) or (B)(4).

It would be nice to size it that way but unfortunately the NEC does not provide that option for EGC's. It does for SSBJ's on something like a transformer secondary which doesn't make much sense. I think that part of the confusion is that the "circuit conductors supplying the equipment" in this case are 5000 amp conductors and that ampacity number that must be used for the EGC in each raceway even if it's larger than the circuit conductors within the raceway.

IMO this section and concept is very messy especially since many metallic raceways don't require an EGC at all. If I use EMT I need to have a 700 kcmil EGC in each of the 12 raceways or I don't need to have any. Seems like a big difference between 12 parallel 700's versus none. :slaphead:

 

[mbrooke]

It would be nice to size it that way but unfortunately the NEC does not provide that option for EGC's. It does for SSBJ's on something like a transformer secondary which doesn't make much sense. I think that part of the confusion is that the "circuit conductors supplying the equipment" in this case are 5000 amp conductors and that ampacity number that must be used for the EGC in each raceway even if it's larger than the circuit conductors within the raceway.

IMO this section and concept is very messy especially since many metallic raceways don't require an EGC at all. If I use EMT I need to have a 700 kcmil EGC in each of the 12 raceways or I don't need to have any. Seems like a big difference between 12 parallel 700's versus none. :slaphead:

Is it possible the code just messed up in wording it?

BTW- didn't earlier codes make an exception that if you had ground fault protection the EGC could be smaller table 250.122?

 

[infinity]

Is it possible the code just messed up in wording it?

BTW- didn't earlier codes make an exception that if you had ground fault protection the EGC could be smaller table 250.122?

If there was an exception it's beyond my recollection but it is possible. We did have undersized EGC's in NM cable for a few code cycles way back when.

 

[mbrooke]

If there was an exception it's beyond my recollection but it is possible. We did have undersized EGC's in NM cable for a few code cycles way back when.

I believe you. But I guess I am left to ask- how common is it to encounter undersized EGC in parallel explications? I ask because I am looking back through old electrical signal lines signed off by engineers approved for construction and Id say about 1/3 of them have EGC less than 250.122. Even I have done it. Either this is a very common mistake (like applying the next size up rule to service conductors) or there is some type of loop hole in the code. Or my computer also slipped into this dimension...

 

[don_resqcapt19]

Is it possible the code just messed up in wording it?

BTW- didn't earlier codes make an exception that if you had ground fault protection the EGC could be smaller table 250.122?

There was a provision for GFP, but it specified that the GFP had to be specifically listed for the purpose. The implied purpose was the protection of the EGC and the 2005 code specified that. There was never a GFP listed for that purpose.

1999 250.122(F)(2) Where ground-fault protection of equipment is installed, each parallel equipment grounding conductor in a multiconductor cable shall be permitted to be sized in accordance with Table 250.122 on the basis of the trip rating of the ground-fault protection where the following conditions are met.
(1) Conditions of maintenance and supervision ensure that only qualified persons will service the installation.
(2) The ground-fault protection equipment is set to trip at not more than the ampacity of a single ungrounded conductor
of one of the cables in parallel.
(3) The ground-fault protection is listed for the purpose.

List item 3 was changed in the 2005 code to read:

The ground-fault protection is listed for the purpose of protecting the equipment grounding conductor.

 

[jtinge]

While reviewing the proposed changes for the next NEC cycle, I see the committee has proposed some changes to 250.122 regarding paralleling of conductors. See attached. Not applicable now, but interesting to understand what may be expected in the next edition.

 

[infinity]

While reviewing the proposed changes for the next NEC cycle, I see the committee has proposed some changes to 250.122 regarding paralleling of conductors. See attached. Not applicable now, but interesting to understand what may be expected in the next edition.

One of our moderators has been trying to get this table changed for many code cycles looks like the CMP might finally agree with him.

 

[mbrooke]

While reviewing the proposed changes for the next NEC cycle, I see the committee has proposed some changes to 250.122 regarding paralleling of conductors. See attached. Not applicable now, but interesting to understand what may be expected in the next edition.

This is gold, and a satisfying read, thank you!